Security papers such as certificates, financial papers, passports, bank notes and stamp papers, etc., suffer from severe issues of counterfeiting and hence need special protection. Several technologies are available to provide anti-counterfeit features to such paper products that vary from use of holograms, watermarks, incorporation of security threads, etc. However, new technologies and new or enhanced anti-counterfeit features are required to stay ahead of counterfeiters who use modern reprographic devices and specialized scanning equipment combined with commercially available high resolution printers to reproduce/simulate/counterfeit the end product.
The use of luminescent substances as a means (or additional means) for guaranteeing the authenticity of security papers, is known. For example, U.S. Pat. No. 4,897,300 A to Michael Boehm discloses a security thread comprising a tear-proof carrier material (e.g., polyester film) printed with luminescent colors that are invisible in normal lighting, but visible when exposed to, for example, ultraviolet radiation. The luminescent colors are provided along the carrier material in successive and overlapping portions, with overlapping areas showing mixed luminescence. This reference teaches at Col. 5, lines 13 to 18, that when an opaque carrier material is used, the carrier material may be printed on both sides, so that orientation of the thread while embedding the thread in paper is not a concern.
The security thread of U.S. Pat. No. 4,897,300 A necessarily requires the use of adhesives to bond the thread into or onto a security document such as paper.
PCT Publication No. WO 2004/025028 A1 discloses fibers that have a plurality of colored fluorescent stripes or regions printed on front and rear sides of the fiber. The stripes or regions are printed in at least two different colors with ultraviolet fluorescent pigments or inks and are only visible under ultraviolet light. This reference teaches that the pigments used for generating the printed stripes do not generally combine well (see page 2, lines 19 to 21, of WO 2004/025028 A1). As such, the overlapping of these stripes is deemed disadvantageous. In fact, this reference teaches that the pigments are selected so that there is no migration of colors into one another. (See, page 14, lines 1 to 5, of WO 2004/025028 A1).
The invention of WO 2004/025028 A1 necessarily requires that the printed regions are registered such that regions on the front and rear sides are registered with one another and in the same color. Such a requirement of registration, which is extremely difficult if not impossible to achieve, introduces several constraints such as limiting the flexibility of manufacturing routes and the need for stringent matching of registrations, which thereby enhances the rejection rates of the products.
There has been a long felt need in the security industry to further protect security papers by providing security features that are difficult to counterfeit and yet remain amenable to flexible and cost-effective manufacturing and to easy authentication by users and issuing authorities.